Current multi-channel LED light sources, such as the Philips system known as “intelligent TLED”, face a major problem of limited space assigned for drivers. This system generates white light by driving red, green and blue LEDs independently. In practice, the green LED makes use of a native blue LED and a green phosphor layer. This system generates warm white light with a very high efficiency. In addition multi-channel LED drivers are also encountered in LED modules or LED luminaries in which different channels are used to generate separate beams for general lighting and task lighting, or where separate LED strings are used to generate cold white or warm white from a single luminaire.
In current systems, the system requires separate drivers for the different LEDs of the module. A problem arises because the available space for the light source drivers is fixed to meet the requirements of traditional light sources, which normally comprise one or at most two channels, with limited functions such as a dimming function. Multi-channel light sources with WW/CW, RGB or more channels have a total peak power as well as a total space consumption which is the combination of the requirements for each channel. In order to compress the driver into a small space, basic performance has to be sacrificed, such as the power factor or efficiency, but this is generally not acceptable to the product designer. There is therefore a need to enable miniaturisation of the driver circuits, without compromising the system performance.
In general, in a multi-channel system, each channel does not always run at maximum power. In many situations, pulse width modulation (PWM) is used to control the power on one channel, meaning that the power on that channel is only drawn during a certain fraction of the time. Therefore, the total maximum power at any particular time is derived from the superposition of the peak power on each channel, when they overlap at the same time.
FIG. 1 shows a conventional multi-channel lighting system driver circuit. Three LED loads 10,11,12 are shown, which may for example have three different colour outputs. Each is driven by a respective driver 20,21,22 which essentially comprises a switch mode power supply (SMPS) or linear driver which implements PWM control. There is a global AC-DC converter 14, which includes power factor correction, and a global controller 16 which is remote to the actual light sources themselves. The global controller 16 provides commands to the local drivers 20,21,22 to control the operation of the LED loads.
The embodiments of the invention relates in particular to the control of the individual drivers 20,21,22.
The known control method is described with reference to FIG. 2, which shows the timing of current being drawn by the three LED loads, named “channel 1”, “channel 2” and “channel 3”.
Each channel uses PWM signal control of the LED load, with the same PWM cycle frequency. At the start of each PWM cycle, each channel starts with a high pulse, the duration of which determines the light output. Thus, all three channels are activated at the same time giving a high power demand at the beginning of each PWM cycle.
For each channel, the maximum power is for example 15W. The control method above gives a total power of 45 W. The output power swing is from 0 Watts to 45 Watts with four possible values of 45 W, 30 W, 15 W, 0 W.
There are three well known ways to ensure sufficient peak power can be delivered:
(i) Use a large output power supply. This may not physically fit.
(ii) Use a high crossover frequency power supply 14, and add a large output capacitor. The AC-DC crossover frequency can for example be around 1 kHz, so that a capacitor can be used to supply 1 ms of additional power dissipation. This approach however reduces the power factor and is not acceptable for mass production of LED systems.
(iii) Use a low crossover frequency power supply to improve the power factor and add a much larger output capacitor. If many LEDs in parallel are assembled inside the luminaire, the power factor must for example be higher than 0.9. In many applications the requirement for the Total Harmonic Distortion (THD) is below 20%. The power factor stage and DC-DC converter stage are typically combined together, and the cross frequency then needs to be below 20 Hz in order to achieve a sufficiently high Power Factor. This means a large capacitor is needed to supply 50 ms of additional power dissipation. In this case, the capacitor can be too large to be installed in the unit.
If the overlap of power supply to each channel can be avoided, the peak power will be derived from the highest peak current in the individual channels. In this case, the total peak power can be greatly reduced, giving space and cost savings for the drivers.
The idea of avoiding overlap of the driving of LEDs has been considered, for example in US 2010/0301764. This document discloses a phase shift method for implementing non-overlapping signals. The individual channels each comprise a delayed version of an input PWM signal with the same duty cycle. In this document, a delay locked loop (DLL) calculates a pulse width of the input PWM signal by a high frequency clock signal (sampling frequency), and generates for each channel a respective phase-shifted PWM signal with the same pulse width but shifted in time by such width. More specifically, it has “the PW mirror duplicates and delays a received PWM signal”. A turn-ON timing of each of the multiple phase-shifted PWM signals follows a turn-OFF timing of a previous PWM signal which is the input PWM signal or a previous one of the multiple phase-shifted PWM signals. Particularly, in FIG. 8B and para [0040], the edge detector 201 detects a rising and/or a falling edge of the received PWM signal. and a pulse width memory circuit 202 memorizes the pulse width of the received PWM signal.